Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

White Paper:
 

Representativeness and Applicability of United Nuclear 

Corporation Air Sampling for Reconstructing Thorium Intakes 

by 

Jason Davis 

1.0 Introduction and Purpose 

United Nuclear Corporation (UNC), located in Hematite, Missouri, manufactured uranium metal 

and uranium metal compounds from natural and enriched uranium for use as nuclear fuel. The 

facility became operational in 1956, initially producing uranium products for use in the naval 

fuel program. For an approximately nine-month period in 1964, UNC also blended thorium 

dioxide powder with uranium dioxide powder to produce fuel pellets for use in fuel assemblies 

for the Elk River Reactor (Pellet Plant Data, 1964; Swallow, 1963; Swallow, 1964a). The 

facility processed un-irradiated uranium scrap for the AEC, recovering enriched uranium for use 

in the nuclear weapons complex. Under the Energy Employees Occupational Illness 

Compensation Program, the site is designated as an Atomic Weapons Employer from 1958 

through 1973; the Residual Radiation period is specified as 1974 through October 2009 (DOE 

Facilities, 2011). 

Although UNC maintained an active urinalysis program for operations workers during the 

nine-month Th/U blending period, the urinalysis samples were analyzed only for uranium 

(Bioassay Results, 1964; AEC, 1964). Instead, UNC-Hematite established a Maximum 

Allowable Concentration (MAC) for thorium work of 2E-11 μCi/ml and verified that exposures 

remained below this level through a routine general area and personal air sampling program 

(Darr, 1964a). Although these air samples are designated ThO2 on the sample log sheets, there is 

no indication that thorium-specific assays were performed in addition to the gross alpha counting 

of the samples (Pellet Plant Data, 1964; Monitoring Surveys, 1964). 

Questions arose during a meeting of the Advisory Board’s Work Group on Uranium Refining 

Atomic Weapons Employers as to whether the available air sampling data are representative of 

the air concentrations to which workers were exposed, and whether counting such data for gross 

alpha rather than specifically assaying for thorium provides a plausible upper-bound for thorium 

exposures. This white paper addresses the representativeness of air sampling measurements 

performed in the Pellet Plant (Building 255) during 1964. This paper specifically addresses the 

expressed concern about the ability to use such air sampling measurements to estimate worker 

intakes to thorium and its daughter products. 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

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Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

2.0 Process Considerations 

Processing of UF6 to UO2 was accomplished in another area of the United Nuclear plant known 

as the Red Room (Darr, 1964b); ThO2 was received from off site. While the thorium was of 

natural abundance, the uranium was enriched to 6.5% U-235, as approved by the U.S. Atomic 

Energy Commission (AEC) (Swallow, 1964a). 

The process material was handled in small batches to limit criticality concerns (UNC Health 

Physics, 1964). Batches consisting of 3% UO2 and 97% ThO2 were blended, granulated to a 

specific size, and dried in a drying oven, all while under a dust-tight hood that exhausted air 

through two MSA filters and into the atmosphere. After drying and inspection, these mixtures 

would be transferred to a pellet press storage hopper under another dust hood. From there, the 

powders would be pressed into pellets and then heated twice to harden. The hardened pellets 

would be graded for size; those that were too large would be ground to the appropriate size on a 

centerless grinder under another dust hood, while those that were too small would be recycled 

back into the process (Leaders, 1958a; Leaders, 1958b). 

3.0 Air Monitoring Details 

An air sampling program was conducted to monitor exposures to airborne radioactive dust, to 

identify process areas that needed better engineering controls or designs, and to identify 

operations that required worker protection to minimize airborne dust exposures. Air samples 

were collected in the Pellet Plant by four general area samplers and 11 breathing zone samplers, 

as shown in Figure 1 (Darr, 1964b). 

In all, 210 air sample measurements were taken between March 27, 1964 and September 23, 

1964. Of these, sample logs indicate that 133 measurements were breathing zone (BZ) samples 

(BZ locations are boxed numbers in Figure 1); 75 were general area (GA) samples (GA locations 

are boxed letters in Figure 1). The remaining two samples were special samples taken in 

response to specific events, such as a spill that occurred during milling (Pellet Plant Data, 1964). 

Although sample logs do not indicate which instrument was used for each sample, 

documentation indicates that UNC possessed a Filtronics, Inc., breathing zone sample head and 

rechargeable Ni-Cd battery, which is labeled “lapel sampler” in the list of health physics 

equipment and instrumentation (Swallow, 1964b). In addition, the UNC Health Physics 

Procedure Manual indicates that, for uranium operations, efforts were made to ensure that 

sampling heads were positioned near the breathing area of the operator (Swallow, 1964b). Given 

that the thorium pelleting operations also involved the use of uranium, there is no reason to 

believe that a deviation from this procedure would have occurred for thorium operations. 

Therefore, it is assumed that all samples listed as BZ were taken in the breathing zone, either 

with a lapel sampler or a sampling head placed in the breathing zone. 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 2 of 9 
 

Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

11 

2 

1 

10 

9 

8 

7 

6 

5 

4 

3 

D 

C 

B 

A 

Source: Darr, 1964b 

Figure 1: BZ and GA Air Sampling Locations in the UNC Pellet Plant in 1964 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 3 of 9 
Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

4.0 Determination of Sample Representativeness 

In the absence of individual monitoring data, EEOICPA allows for the use of workplace area 

monitoring data to estimate personnel doses. In such a case, workplace area monitoring data are 

understood to include both general area air sampling results as well as breathing zone air 

sampling results (42 C.F.R. pt. 82). However, if air samples are to be used for determining 

intake, correct interpretation of the data is important and should include some analysis of the 

extent to which samples represent the air inhaled by the workers (NUREG-1400). 

As stated in Section 3.0, 133 of the 210 air samples covering the thorium operational period were 

breathing zone samples. These samples were taken in breathing zones near hood openings or in 

other areas where airborne contamination was anticipated to be the highest. These samples are 

considered representative of maximum intakes for workers while performing operations in hoods 

or while handling thorium or thorium-contaminated equipment. The 75 general area air samples 

were taken at four locations throughout the process area. These samples would be representative 

of breathing air for workers while they were located in the process area, but not while actively 

performing operations in hoods or handling thorium or thorium-contaminated equipment. 

Therefore, all 210 samples together are representative of the overall distribution of exposure 

potential a worker may have received in the process area, where the upper portion of the 

distribution is used for routinely- exposed workers and the median would be representative of the 

non-routinely-exposed workers. 

5.0 Evaluation of Gross Alpha Counting 

The air samples were counted only for gross alpha activity rather than undergoing a specific 

radionuclide assay (Monitoring Surveys, 1964). Table 1 presents a comparison of the dose 

coefficients per unit of inhaled activity (ICRP, 1994) for 100% U-234, the United Nuclear fuel 

mixture, natural thorium, and freshly-separated thorium. The raw ICRP 68 dose coefficients for 

each of the respective isotopes are given in Table 2. For the 100% U-234 calculation, the ICRP 

suggestion of a Type S material for uranium and thorium oxides was taken. The fuel mixture 

coefficients assume that the fuel was 3% UO2 and 97% ThO2 by weight and that the uranium 

was enriched to 6.5% U-235 by weight. As a result of the enrichment process, it was assumed 

that the fraction of U-234 increased to 0.0555% by weight. Th-232 and Th-228 were assumed to 

be in secular equilibrium and the relative contribution of each isotope to the total alpha activity 

was determined. The calculated activity ratio was used to weight the dose coefficients for each 

radionuclide. 

For natural thorium, an assumption of secular equilibrium was made and contributions from 

Th-232, Th-228, and Ra-224 are included in the dose coefficients. Finally, the dose coefficients 

for freshly-separated thorium assume that sufficient time has not elapsed to allow for the 

in-growth of radium and other daughter products. Therefore, the alpha contribution to total dose 

from this scenario would be exclusively the result of decaying Th-232 and Th-228. 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 4 of 9 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

In all cases, the uranium results in lower dose to each organ per unit intake than thorium. It is 

also important to note that uranium doses are assessed via urinalysis records. Therefore, the 

assumption that all alpha counts present on air samples are the result of thorium decay does not 

reduce the assessed dose due to uranium intakes. Therefore, the assumption that all of the 

detected alpha activity is due to the decay of thorium is claimant-favorable. 

Table 1: Inhalation Dose Coefficient Comparison: Enriched uranium, UNC Fuel Mixture, Natural 

Thorium, and Freshly-Separated Thorium (Sv/Bq) 

(ICRP 68) 

Organ Radionuclide 

U-234 UNC Fuel Mixturea Natural Thoriumb Th-232/Th-238c 

Adrenals 8.68E-09 1.11E-07 1.50E-07 2.24E-07 

Urinary Bladder 8.61E-09 1.08E-07 1.46E-07 2.17E-07 

Bone Surface 2.68E-07 3.63E-05 5.09E-05 7.60E-05 

Brain 8.61E-09 1.08E-07 1.46E-07 2.17E-07 

Breast 8.68E-09 1.11E-07 1.50E-07 2.24E-07 

Esophagus 8.69E-09 1.11E-07 1.51E-07 2.24E-07 

Stomach 9.14E-09 1.11E-07 1.51E-07 2.23E-07 

Small Intestine 9.91E-09 1.12E-07 1.52E-07 2.24E-07 

Upper Large Intestine 1.69E-08 1.27E-07 1.77E-07 2.48E-07 

Lower Large Intestine 3.20E-08 1.64E-07 2.37E-07 3.10E-07 

Colon 2.30E-08 1.42E-07 2.02E-07 2.74E-07 

Kidneys 9.91E-08 7.00E-07 9.12E-07 1.36E-06 

Liver 3.66E-08 7.36E-07 1.01E-06 1.51E-06 

Muscle 8.64E-09 1.11E-07 1.50E-07 2.23E-07 

Ovaries 8.61E-09 3.38E-07 4.68E-07 7.00E-07 

Pancreas 8.65E-09 1.11E-07 1.50E-07 2.23E-07 

Red Marrow 2.78E-08 1.69E-06 2.37E-06 3.52E-06 

Extrathoracic Airways 7.40E-05 1.15E-04 1.09E-04 1.60E-04 

Lungs 4.03E-05 8.94E-05 1.03E-04 1.44E-04 

Skin 8.62E-09 1.08E-07 1.46E-07 2.17E-07 

Spleen 8.65E-09 1.11E-07 1.50E-07 2.23E-07 

Testes 8.61E-09 3.38E-07 4.68E-07 7.00E-07 

Thymus 8.69E-09 1.11E-07 1.51E-07 2.24E-07 

Thyroid 8.63E-09 1.11E-07 1.50E-07 2.23E-07 

Uterus 8.61E-09 1.08E-07 1.46E-07 2.17E-07 

Source: ICRP 68, Dose Coefficients for Intakes of Radionuclides by Workers (ICRP, 1994) a The UNC fuel mixture assumes 3% UO2 and 97% ThO2 by weight and assumes uranium has been enriched to 

6.5% U-235 b Natural thorium dose coefficients are determined assuming a secular equilibrium between Th-232, Th-228, and 

Ra-224. c The Th-232/Th-238 mixture assumes thorium was very recently separated chemically and sufficient time has 

not elapsed to allow for the in-growth of radium or other decay products. 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

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Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

Table 2: Inhalation Dose Coefficient Comparison: U-234, Th-232, Th-238, and Ra-224 (Sv/Bq) 

(ICRP 68) 

Radionuclide > U-234 Th-232 Th-228 Ra-224 

Half Life > 2.445E5y 1.405E10y 1.9131y 3.66d 

Type > F M S M S M S S 

AMAD > 5 5 5 5 

Organ f1 > 0.02 0.02 0.002 0.0005 0.0002 0.0005 0.0002 0.2 

Adrenals 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.70E-08 4.19E-09 

Urinary Bladder 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.00E-07 7.80E-07 3.40E-08 4.28E-09 

Bone Surface 1.10E-05 2.70E-06 2.70E-07 1.50E-03 1.40E-04 2.80E-04 1.20E-05 7.27E-07 

Brain 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.00E-07 7.70E-07 3.40E-08 4.13E-09 

Breast 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.70E-08 4.17E-09 

Esophagus 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.80E-08 4.20E-09 

Stomach 3.90E-07 9.40E-08 9.20E-09 4.40E-06 4.10E-07 7.70E-07 3.60E-08 6.04E-09 

Small Intestine 3.90E-07 9.50E-08 1.00E-08 4.40E-06 4.10E-07 7.80E-07 3.70E-08 8.29E-09 

Upper Large Intestine 3.90E-07 1.00E-07 1.70E-08 4.50E-06 4.30E-07 1.10E-06 6.50E-08 3.52E-08 

Lower Large Intestine 4.00E-07 1.10E-07 3.20E-08 4.60E-06 4.80E-07 1.70E-06 1.40E-07 9.22E-08 

Colon 3.90E-07 1.10E-07 2.30E-08 4.50E-06 4.50E-07 1.40E-06 9.70E-08 5.97E-08 

Kidneys 4.00E-06 9.80E-07 1.00E-07 2.30E-05 2.40E-06 7.30E-06 3.20E-07 1.76E-08 

Liver 1.50E-06 3.70E-07 3.70E-08 2.30E-05 2.50E-06 1.20E-05 5.10E-07 2.30E-08 

Muscle 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.50E-08 4.18E-09 

Ovaries 3.90E-07 9.40E-08 8.70E-09 1.30E-05 1.30E-06 2.30E-06 1.00E-07 4.43E-09 

Pancreas 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.60E-08 4.18E-09 

Red Marrow 1.10E-06 2.80E-07 2.80E-08 5.70E-05 6.10E-06 2.20E-05 9.40E-07 6.96E-08 

Extrathoracic Airways 3.90E-07 1.20E-05 7.50E-05 1.50E-05 1.10E-04 5.80E-05 2.10E-04 6.17E-06 

Lungs 4.00E-07 1.60E-05 4.10E-05 1.60E-05 7.70E-05 1.30E-04 2.10E-04 2.12E-05 

Skin 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.00E-07 7.70E-07 3.40E-08 4.13E-09 

Spleen 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.60E-08 4.17E-09 

Testes 3.90E-07 9.40E-08 8.70E-09 1.30E-05 1.30E-06 2.40E-06 1.00E-07 4.13E-09 

Thymus 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.80E-08 4.20E-09 

Thyroid 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.10E-07 7.70E-07 3.50E-08 4.15E-09 

Uterus 3.90E-07 9.40E-08 8.70E-09 4.40E-06 4.00E-07 7.70E-07 3.40E-08 4.24E-09 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 6 of 9 
Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

6.0 Method for Assessing Doses from Intakes of Thorium 

The 210 air samples collected in the Pellet Plant during thorium operations in 1964, which 

include a combination of breathing zone and general area samples, are believed to be a 

reasonable representation of airborne thorium concentrations inside the plant during that time. A 

method for assessing doses resulting from thorium intakes is described in DCAS-TKBS-0008. It 

employs the data from the 210 air samples to determine the parameters of a lognormal 

distribution. Operators who routinely handled thorium or operated thorium-processing 

equipment are assigned the 95th percentile of this distribution as a constant. Supervisors, 

laborers, and other personnel who routinely entered the processing area but did not routinely 

handle thorium are assigned the full distribution (geometric mean with GSD as a lognormal 

distribution). Personnel who did not routinely enter the area are assigned the median value of the 

distribution as a constant. However, DCAS-TKBS-0008 should be modified to recommend the 

assignment of thorium assuming a thorium mixture. 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 7 of 9 
Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

References 

42 C.F.R. pt. 82, Methods for Radiation Dose Reconstruction Under the Energy Employees 

Occupational Illness Compensation Program Act of 2000; Final Rule; May 2, 2002; SRDB Ref 

ID: 19392 

AEC, 1964, Report of Annual Health Protection Review of United Nuclear Corporation 

Hematite, Missouri, July 23-24, 1964; United States Atomic Energy Commission (AEC); August 

20, 1964; SRDB Ref ID: 62312 

Bioassay Results, 1964, Various 1960 Handwritten Bioassay Results for Hematite Workers; 

United Nuclear Corporation; various dates throughout 1964; SRDB Ref ID: 62390 

Darr, 1964a, Health Physics Considerations for Handling and Processing of ThO2-UO2, 

correspondence to E. F. Sanders; D. G. Darr; May 7, 1964; SRDB Ref ID: 62430 

Darr, 1964b, Air Concentrations at Drying Oven and Agglomeration Hood, Pellet Plant, 

correspondence to E. F. Sanders (United Nuclear Corporation); June 19, 1964; SRDB Ref ID: 

62431, pdf pp. 2-4 

DCAS-TKBS-0008, Technical Basis Document for the United Nuclear Corporation – Hematite, 

Missouri, Rev. 0; National Institute for Occupational Safety and Health – Division of 

Compensation Analysis and Support; March 21, 2011; SRDB Ref ID: 93712 

DOE Facilities, 2011, Facility List Database, Energy Employees Occupational Illness 

Compensation Program; U.S. Department of Energy, Office of Health, Safety and Security; 

http://www.hss.energy.gov/healthsafety/fwsp/advocacy/faclist/findfacility.cfm 

DOE G 441.1-12, Radiation Safety Training Guide for Use with Title 10, Code of Federal 

Regulations, Part 835, Occupational Radiation Protection, U.S. Department of Energy; 1999; 

Publicly available 

ICRP, 1994, Dose Coefficients for Intakes of Radionuclides by Workers, International 

Commission on Radiological Protection (ICRP); Publication 68; Pergamon Press, New York, 

N.Y.; 1994; Publicly available 

Leaders, 1958a, Special Nuclear Materials License No. SNM-33: Pelleting Process, 

correspondence to L. Johnson (United States Atomic Energy Commission); W. M. Leaders, 

Mallinckrodt Chemical Works; February 20, 1958; SRDB Ref ID: 56223 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 8 of 9 
Rev. 0 DRAFT UNC Thorium Air Samples November 14, 2011 

Leaders, 1958b, Special Nuclear Materials License No. SNM-33: Pelleting Process, 

correspondence to L. Johnson (United States Atomic Energy Commission); W. M. Leaders, 

Mallinckrodt Chemical Works; December 10, 1958; SRDB Ref ID: 56378 

Monitoring Surveys, 1964, Monitoring Surveys Effective February 18, 1964; United Nuclear 

Corporation; February 18, 1964; SRDB Ref ID: 57004 

NUREG-1400, Air Sampling in the Workplace, E. E. Hickey, G. A. Stoetzel, D. J. Strom, G. R. 

Cicotte, C. M. Wiblin, S. A. McGuire; U.S. Nuclear Regulatory Commission; 1993; Publicly 

available 

Pellet Plant Data, 1964, Pellet Plant Station Alpha Activity Data Sheets, United Nuclear 

Company; various dates throughout 1964; SRDB Ref ID: 62343 

Swallow, 1963, Health Physics Problems Associated with Thoria, correspondence to A. 

Edelmann (Nuclear Science & Engineering); October 31, 1963; SRDB Ref ID: 62315 

Swallow, 1964a, SNM-33 Amendment to Permit Blending Fully Enriched UO2 with Natural 

ThO2 in the Pellet Plant, correspondence to Mr. Donald Nussbaumer (U.S. Atomic Energy 

Commission); March 31, 1964; SRDB Ref ID: 56730 

Swallow, 1964b, United Nuclear Corporation, Chemicals Operation Health Physics Manual; L. 

J. Swallow, United Nuclear Corporation; August 15, 1963, Revised November 25, 1964; SRDB 

Ref ID: 62183 

This is a working document prepared by NIOSH or its contractor for use in discussions with the ABRWH or its Working Groups 

or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or ABRWH (or their technical 

support and review contractors) positions unless specifically marked as such. This document represents preliminary positions 

taken on technical issues prepared by NIOSH or its contractor. 

Page 9 of 9